1. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a cluster-based cooperative communication system and method for mitigating inter-cell interference effectively.
2. Description of the Related Art
Inter-cell interference is a significant problem to be addressed in a Multiple Input Multiple Output (MIMO) communication system. Recently, research has been conducted on a cooperative base station system for more actively mitigating the inter-cell interference. Pre-coding Matrix Index (PMI) restriction (or PMI recommendation) is known as an efficient technique for compressing an amount of overhead incurred in the cooperative base station system.
FIG. 1 is a schematic diagram illustrating a conventional cooperative base station system.
In FIG. 1, the cooperative base station system includes a first base station 111 defining a cell 121, a second base station 211 defining a cell 221, a first mobile station 101, and a second mobile station 201. The first mobile station 101 is located at a cell boundary area where signal strength from each of the first and second base stations 111 and 211 is weak, and the second mobile station 201 is located in the cell 221 of the second base station 211.
In this case, a signal transmitted from the second base station 211 to the second mobile station 201 is likely to strongly influence the first mobile station 101. More particularly, when the first and second mobile stations 101 and 201 are allocated the same time-frequency resource, the signal from the second base station 211 to the second mobile station 201 must be a dominant interference to the first mobile station 101. Accordingly, in a wireless communication system of which frequency reuse factor is 1, the performance of a mobile station is significantly degraded at a cell edge.
The PMI restriction is a method to address the aforementioned problem in the MIMO communication system. The first mobile station 101 estimates downlink channels of the first and second base stations 111 and 211, calculates the pre-coding matrix (or vector) of the second base station 211 which is potentially interfering with the first mobile station 101, and reports the Pre-coding Matrix Index (PMI) to the first base station 111. The first base station 111 sends the PMI to the second base station 211 such that the second base station 211 restricts the use of the PMI informed by the first base station 111.
The PMI restriction process is advantageous in mitigating the inter-cell interference to the cell edge mobile station. However, its performance is limited so as to control the interference from a single cell. Although it can be considered to propose an extended PMI restriction technique, the extended PMI restriction technique is also limited in performance to a preset number of cells.
Since cells in a cellular system cannot be completely isolated from each other and must coexist, they naturally interfere with each other. Furthermore, each cell can be divided into sectors such that it is rare for a mobile station to be placed in a situation in which it must consider only one or two cells (or sector), whereby it is difficult to expect sufficient inter-cell interference mitigation performance of the PMI restriction technique limited in number of neighbor cells. This means that if a base station fails to cooperate with a neighbor base station as a potentially dominant inter-cell interference source, then a minimal inter-cell interference mitigation effect can be expected. In a cellular system in which cooperative base station groups are preset, there exists an overlapping area between the base station groups, and thus the mobile station located at the overlapping area does not benefit from the cooperative base station system.